Ferritin, the iron storage protein used by a wide range of organisms, serves the dual functions of iron detoxification and storage; it catalyzes the oxidation of the toxic Fe2+ ions in the cells to the less toxic Fe3+ ions (ferroxidation) and stores the oxidized Fe3+ ions within its protein shell in a mineral form similar to ferrihydrite (mineralization). Recent results from rapid-freeze-sequence Mvssbauer have demonstrated the formation of a well defined peroxodiferric intermediate during the ferroxidase reaction in the initial states of fast ferritin mineralization. We are using EXAFS to characterize the structures of these intermediates. The comibination of EXAFS with Mvssbauer allows us to quantitate the amount of the peroxidatic species present in our EXAFS samples. Preliminary measurements show a novel Fe-Fe interaction in this intermediate.